The present invention relates to an exhaust gas purifying facility with a nitrogen oxides absorption-reduction catalyst (hereinafter referred to as NOx absorption-reduction catalyst) for purifying nitrogen oxides (NOx) in exhaust gas by absorbing NOx when the air/fuel ratio of exhaust gas of the internal combustion engine is lean, and by discharging and reducing NOx when it is rich.
More specifically, the present invention relates to an exhaust gas purifying facility that may be applied to all the lean burn vehicles and is provided with an NOx absorption-reduction catalyst and an NOx sensor so that NOx absorption-reduction catalyst can be regenerated every appropriate period of time while using the detected value of the NOx sensor for the operation of an engine employing a lean-burn system.
As one of effective technologies for improving fuel consumption of gasoline engines, there is a lean-burn system. However, this system has a problem in that three-way catalyst that is used conventionally cannot be used for purification of NOx since a plenty of oxygen is contained in exhaust gas. Accordingly, a development of catalysts that can purify NOx even when exhaust gas is in the hyperoxia atmosphere has been made.
As a consequent, an NOx absorption-reduction catalyst in which NOx is absorbed in basic element such as barium (Ba) or the like in the region of lean air/fuel ratio and then the absorbed NOx is desorped (discharged) and reduced in the region of rich air/fuel ratio has developed recently and already commercialized.
As an example of the conventional NOx absorption-reduction catalysts, there is an NOx absorption-reduction catalyst having a NOx absorbing substance, as stated in Japanese Patent Publication No. 2600492. The arrangement of active metal on the surface of the supporting layer of the NOx absorption-reduction catalyst and the mechanism of reduction and purification of NOx are shown in FIG. 4.
The NOx absorption-reduction catalyst 2 is formed in such a manner that a catalytic active metal 3 and a NOx absorbing substance (R) 4 having NOx absorbing feature are supported on the supporting layer 5 formed on the support 6. The supporting layer 5 is formed of a porous coating material such as porous zeolite or alumina (Al2O3), or the like. The catalytic active metal 3 is formed of platinum (Pt) having an oxidation catalytic capability. NOx absorbing substance (R) 4 is formed of potassium (K), barium (Ba), lanthanum (La) or the like.
The NOx absorption-reduction catalyst 2 absorbs NOx and discharges and purifies NOx depending on the concentration of oxygen in exhaust gas, or the concentration of carbon monoxide. In other words, the NOx absorption-reduction catalyst 2 exercises two features of absorption of NOx, and discharge and purification of NOx.
The NOx absorption-reduction catalyst 2 uses its oxidation feature of catalytic metal 3 such as platinum to oxidize nitric monoxide (NO) in exhaust gas into nitrogen dioxide (NO2) by oxygen in exhaust gas as shown in FIG. 4(a) under the operation of lean air/fuel ratio in which oxygen (O2) is contained in exhaust gas as in the case of normal diesel engines or lean-burn gasoline engine, and the like. The nitrogen dioxide reacts with barium 4 or the like, which is a NOx absorbing substance, to form nitrate (for example Ba(NO3)2 and the like) for absorption. Occlusion of nitrogen dioxide purifies NOx in exhaust gas.
When a state in which nitrogen dioxide is being absorbed continues, barium 4 or the like that has NOx absorbing feature is entirely changed into nitrate, thereby losing NOx absorbing capability. Therefore, in order to recover NOx absorbing capability, the operating condition of the engine is changed into a rich spike that generates exhaust gas called a rich spike gas to deliver the rich spike gas to the NOx absorption-reduction catalyst 2. The rich spike gas is exhaust gas of high temperature generated in rich air/fuel ratio (the theoretical air/fuel ratio or an air/fuel ratio close to the theoretical air/fuel ratio) operation in which the concentration of oxygen (O2) contained therein is close to zero.
As shown in FIG. 4(b), owing to the rich spike, oxygen (O2) in exhaust gas is eliminated, and when the temperature of exhaust gas rises, nitrate in which NOx is absorbed discharges nitrogen dioxide (NO2) and returned into barium (Ba), which is the original state. Since no oxygen exists in exhaust gas, discharged nitrogen dioxide is reduced and purified into water (H2O), carbon dioxide (CO2), and nitrogen (N2) by the reducing agents such as carbon monoxide (CO), carbon hydride (HC), Hydrogen (H2), or the like in exhaust gas on the catalytic metal 3 such as platinum (Pt) having oxidation capability.
In case where exhaust gas is purified by the use of NOx absorption-reduction catalyst, the absorption of NOx under the lean-burn operation, and discharge and purification of NOx under rich-burn operation (rich spike) are repeated to continuously purify NOx exhausted from the engine.
On the other hand, since the operation of engine in rich air/fuel ratio that generate a rich spike gas for reduction of NOx causes deterioration of fuel consumption, it is necessary to keep the rich spike as low as possible. Therefore, when the method of executing a rich spike regularly by the use of a timer is employed, NOx emission changes significantly depending on the number of revolution of the engine or a load, and thus the change of NOx emission cannot be supported satisfactorily, thereby coming to the conclusion that this method is not effective.
From these reasons, in the related art, the control apparatus of the computer for controlling the engine called ECM is used for controlling a rich spike according to the control flow shown in FIG. 5. In other words, the concentration of NOx at each moment is calculated from entered load and the number of revolutions by the use of NOx concentration map that is entered in advance. As a next step, the emission of NOx (NOxc) for each operating condition of the engine is calculated by the use of calculated NOx concentration and entered intake air mass (Q), and then the emission of NOx (NOxc) is integrated. When the integrated value of NOx (NOxmass) reaches a prescribed threshold value (first reference value: NOxSL), a rich spike is executed.
However, in the related art, control is carried out with the prescribed threshold value (first reference value: NOXSL) fixed despite of the fact that the NOx absorbing capability of the catalyst is lowered with deterioration of the catalyst over time. Therefore, there is a problem in that a rich spike cannot be executed at the moment most suitable for recovering the NOx absorbing capability.
In addition, the NOx absorption-reduction catalyst is susceptible to deterioration due to sulfur poisoning, which may lower the NOx absorbing capability of the catalyst. Therefore, when a rich spike is executed according to the normal control of a rich spike, fuel consumption may be deteriorated.
In order to prevent the adverse effect caused by sulfur poisoning, it is required to promote desorption of sulfur by performing the operation in regeneration mode in which the lean-burn operation is prohibited on a regular basis and the engine is operated in a rich air/fuel ratio. However, since this regard is not considered in the exhaust gas purifying apparatus in which the NOx absorption-reduction catalyst of the related art is used, there is a problem in that the operation in regeneration mode in rich air/fuel ratio cannot be performed.
The progress of deterioration of the catalyst by sulfur poisoning varies depending on the description of the fuel or the history of the operation of the engine. Therefore, in order to know the degree of deterioration of the catalyst, it is required to monitor the state of deterioration of the catalyst.
With such circumstanced in view, it is an object of the present invention to provide an exhaust gas purifying apparatus comprising a nitrogen oxides absorption-reduction catalyst for absorbing, as well as reducing and purifying NOx in exhaust gas, wherein the value detected by the NOx sensor is used to monitor the variations in NOx absorbing capability (the amount that can be absorbed) with deterioration of the catalyst, so that a rich spike can be introduced at appropriate moments and the purifying performance with respect to NOx in exhaust gas can be improved outstanding as a whole.
It is another object to provide an exhaust gas purifying apparatus in which the value detected by the NOx sensor is used to monitor the lowering of the NOx absorbing capability caused by sulfur poisoning so that the operation in regeneration mode, or the rich air/fuel ration operation, is executed at appropriate moments to promote desorption of sulfur from the catalyst.
The exhaust gas purifying apparatus including a nitrogen oxides absorption-reduction catalyst for achieving the objects described above comprises a nitrogen oxides absorbing substance disposed in the exhaust passage of the internal combustion engine for absorbing nitrogen oxides when the air/fuel ratio is lean and discharging nitrogen oxides when the air/fuel ratio is theoretical air/fuel ratio or rich, and a precious metal catalyst, and a control apparatus for computing the integrated value of NOx from the engine load, the number of revolutions of the engine, and the intake air mass, so that a rich spike is executed when the integrated value of NOx exceeds the predetermined first reference value, characterized in that a NOx sensor is provided downstream from the nitrogen oxides absorption-reduction catalyst, and in that the control apparatus corrects the predetermined first reference value when the value detected by the NOx sensor immediately after the rich spike is executed exceeds the predetermined second reference value.
The rich spike is a special operational control of the engine in which the air/fuel ratio is temporarily shifted to rich side to supply exhaust gas of low oxygen concentration for discharging and reducing NOx to regenerate the NOx absorbing capability. The rich spike is executed for one to two seconds of period before the amount of the NOx absorption is saturated.
In this exhaust gas purifying apparatus including nitrogen oxides absorption-reduction catalyst, an NOx sensor for detecting the concentration of NOx is mounted immediately downstream of the NOx absorption-reduction catalyst to monitor the state of deterioration of the catalyst from the concentration of NOx after the NOx absorption-reduction catalyst has passed. The slice level, which is the first reference value with respect to the integrated value of the NOx emission for determining whether or not a rich spike is to be introduced, may be changed according to the NOx absorbing capability (the amount that can be absorbed) by the used of the measured value of the NOx concentration.
Therefore, the frequency of the introduction (occurrence) of the lowering of the NOx absorbing capability may be increased, and the rich spike can be introduced at appropriate moments. In other words, a rich spike can be executed at appropriate moments by correcting the first reference value for determining the timing of the introduction of a rich spike according to the NOx absorbing capability.
The exhaust gas purifying apparatus including the nitrogen oxides absorption-reduction catalyst is constructed in such a manner that the control apparatus performs the operation in regeneration mode in the rich air/fuel ratio when the predetermined first reference value is smaller than the predetermined third reference value.
The operation in regeneration mode refer to an operation for regenerating the NOx absorbing capability of the NOx absorption-reduction catalyst, in which the operation of theoretical air/fuel ratio is continuously executed, for example, for 10 to 30 minutes with the lean-burn operation prohibited.
With this control, the operation in regeneration mode can be introduced appropriately using the first reference value that is to be corrected according to the NOx absorbing capability as a criterion for assessment of the introduction of the operation in regeneration mode.
Since this operation in regeneration mode enables regeneration of the NOx absorbing capability of the catalyst, the deterioration of fuel consumption caused by frequent execution of the rich spike due to sulfur poisoning may be prevented. The increase in frequency of the rich spike is caused by lowering of the NOx absorbing capability of the catalyst as a result of growing sulfur poisoning of the catalyst due to the long-term operation of the engine.
In addition, the exhaust gas purifying apparatus including a nitrogen oxides absorption-reduction catalyst is constructed in such a manner that the control apparatus determines that the nitrogen oxides absorption-reduction catalyst is in the abnormal state when the value detected by the NOx sensor immediately after executing the operation in regeneration mode exceeds the predetermined fourth reference value. In this arrangement, the abnormality of the NOx absorption-reduction catalyst can be determined, thereby urging the operator an appropriate countermeasure.
Therefore, in the exhaust gas purifying apparatus having the nitrogen oxides absorption-reduction catalyst of the present invention, the NOx absorbing capability of the nitrogen oxides absorption-reduction catalyst can be figured out, and thus the NOx purification rate can always be maintained at a high level. Since the number of the introductions of the rich spike may be maintained at low as possible, the deterioration of fuel consumption may be prevented.